The electric double layer capacitor is a condenser utilizing an electric double layer formed between an electrode and an electrolyte, in which a cycle of electrically adsorbing an ion on a surface of the electrode from the electrolyte is a charge-discharge cycle, so that it is different from a battery in which a cycle of oxidation-reduction reaction accompanied with a mass transfer is a charge-discharge cycle. Therefore, the electric double layer capacitor is excellent in the instant charge-discharge characteristics as compared with the battery, and also it is not accompanied with the chemical reaction and hence the instant charge-discharge characteristics are not substantially deteriorated even in the repetition of the charge-discharge. Also, a simple and cheap electric circuit is sufficient in the electric double layer capacitor because there is no overvoltage in the charge-discharge. Further, it has many merits that the residual capacity is easily understandable and the temperature durability is good over a wide temperature range of −30 to 90° C. and there is no pollution and the like as compared with the battery, so that it recently comes under the spotlight as an earth-conscious and new energy-storing product. Furthermore, since the electric double layer capacitor has the above-mentioned characteristics, it also comes under the spotlight as a power source for energy regeneration or engine start-up in an electric automobile, a fuel cell vehicle and a hybrid electric automobile.
The electric double layer capacitor is an energy-storing device comprising positive and negative electrodes and an electrolyte, in which positive and negative charges are oppositely arranged in a contact interface between the electrode and the electrolyte at an interval of a very short distance to form an electric double layer. Therefore, the electrolyte plays a role as an ion source for the formation of the electric double layer, so that it is an important substance dominating the basic characteristics of the energy-storing device likewise the electrode. As the electrolyte, there have hitherto been known an aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte and the like. From a point of improving the energy density of the electric double layer capacitor, the non-aqueous electrolyte capable of setting a high operating voltage particularly comes under the spotlight and are putting into practical use. As the non-aqueous electrolyte is now practiced a mixed solution obtained by dissolving a solute (support salt) such as (C2H5)4P.BF4, (C2H5)4N.BF4 or the like in an aprotic organic solvent having a high dielectric constant such as a carbonate (ethylene carbonate, propylene carbonate, etc), γ-butyrolactone or the like.
However, since the aprotic organic solvent is low in the flash point, there is a high risk of firing, for example, when the electric double layer capacitor ignites due to heat generation or the like. Moreover, there is a high risk that the aprotic organic solvent is vaporized and decomposed to generate a gas as the electric double layer capacitor generates heat, or the generated gas and heat cause explosion and ignition of the electric double layer capacitor.